Satellites are used for a wide variety of communications ranging from telephone communication to high speed data communications. Satellite communications systems are useful alternatives to conventional terrestrial communications systems, such as land lines, fiber optic lines, microwave repeaters, and the like. A variety of satellites in various orbits are used to provide the different forms of communication. For example, it is common to use communications satellites in geostationary orbits. Such geostationary orbits require the insertion of satellites at a location approximately 22,300 miles from earth. Geostationary satellites have a circular orbit that lies in the plane of the earth's equator and turns about the polar axis of the earth in the same direction and with the same period as the rotation of the earth such that the satellite is in a fixed position relative to the surface of the earth. While being in a fixed position is an advantage, geostationary satellites are extremely expensive due to the high cost of insertion into a geostationary orbit. Furthermore, earth-based stations communicating with a geostationary satellite require high power transmission levels to effectively communicate with a satellite 22,300 miles away. In addition, transmission delays, due to the time required for radio signals to propagate up to the satellite and back to earth, are a significant problem with geostationary satellite communications systems.
To overcome the disadvantages of transmission delay associated with communications using a geostationary satellite communication system, a series of low-Earth orbit (LEO) satellites may be inserted into non-geosynchronous orbits. With a low-Earth orbit, the satellites move relative to the earth's surface. Several proposed satellite communication systems use a constellation of LEO satellites to form a network capable of communicating with an earth-based station. Because the LEO satellites move relative to the earth's surface, a conventional earth-based station for communicating with the LEO constellation must have an antenna capable of tracking each satellite as it moves overhead. With proper antenna tracking, a given earth-based station can communicate with a particular satellite for a period that depends on the altitude of each satellite. For example, as a current satellite passes out of range of an earth-based station, the earth-based station will subsequently switch to a new satellite which comes within reception range.
To accommodate such switching without interrupting data communication, the antenna for the earth-based station must quickly switch from the current satellite to the new satellite. Unfortunately, antenna designs for an earth-based station capable of switching between satellites are often complex and very expensive. For example, an electronically steerable phased array antenna may be used as the antenna for the earth-based station. The phased array antenna tracks a first satellite before quickly switching to lock onto a second satellite before the first satellite passes out of range. However, as those skilled in the art can appreciate, phased array antennas are extremely expensive.
A lower cost approach utilizes two conventional satellite dish antennas to track and switch between two satellites. A first of the two satellite dish antennas tracks the first satellite while the second antenna searches and locks onto the second satellite when it comes into range. The earth-based station then switches from the first antenna to the second antenna to allow communications with the second satellite. The first satellite dish antenna subsequently repositions itself to detect yet a third satellite that will come into range before the second satellite passes out of range of the earth-based station. Thus, the earth-based station alternates use of each antenna for communications, with the active antenna tracking the satellite currently in range while the inactive antenna searches for the next satellite that will come into range.
Although a conventional satellite dish antenna is less expensive than a phased array antenna, the use of two separate antennas increases the overall cost and complexity of the earth-based station. It can therefore be appreciated that there is a significant need for a low-cost earth-based station that can effectively communicate with low-Earth orbit satellites. The present invention offers these and other advantages, as will be apparent from the following description and accompanying figures.